Electromagnetic well logging, and more specifically, electromagnetic induction well logging, may be used to determine electrical properties of earth formations. Reservoir rocks holding hydrocarbons are typically much less electrically conductive than non-reservoir rocks holding interstitial water, and therefore, measuring formation conductivity or resistivity may provide a useful guide to the presence of oil, gas, or water. Such measurements can also be used to find locations of bed boundaries between rock formations. Knowledge of the locations of bed boundaries relative to the drilling trajectory while drilling may be used to perform geosteering to adjust a borehole trajectory.
Transient electromagnetic induction well logging may use a reversible magnetic dipole to generate transient currents in earth formations in order to study the electrical properties of the earth formations. For example, a subsurface logging instrument may consist of a magnetic transmitter and an induction magnetic sensor receiver. The transmitter may act as a magnetic dipole and be a switchable magnet source or an induction coil driven by a switched electric current. This instrument may be lowered and utilized for electromagnetic induction well logging. When the magnetic dipole is rapidly switched on or off, transient currents are induced in the surrounding rock formations. These currents diffuse outward and experience changes as the currents intersect variations in rock resistivity. As the currents diffuse outward, they in turn induce magnetic fields that may be detected in the receiver sensor. Processing of the detected signals may produce a profile of the formation.
Existing induction sensors typically use ferrites as part of their magnetic core. Ferrites are a ceramic compound consisting of a mixed oxide of iron and one or more other materials. Ferrites have ferromagnetic properties. Induction sensors use core materials that are “soft” magnetic materials, with low coercivity. For a low coercivity material, the magnetic flux is induced at very low magnetic field strength and the material retains very little magnetization. In contrast, hard magnetic materials require a high strength magnetic field to alter their magnetic flux and hard magnetic materials retain magnetization. Hard magnetic materials are used for permanent magnets.
An increase in the receiver sensor's sensitivity to transient signals may lead to an increase in accuracy. There remains a need for better techniques to implement electromagnetic induction well logging, and more particularly, a need to increase the sensor's sensitivity.